Ancient Gneiss Complex Research Articles

Architectural and Compositional Diversity of Early Earth Ocean Floor Evidenced by the Paleoarchean Nondweni Greenstone Belt, South Africa

Abstract The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

A Mesoarchean Large Igneous Province on the Eastern Kaapvaal Craton (Southern Africa) Confirmed by Metavolcanic Rocks from Kubuta, Eswatini

Abstract Mesoarchean magmatism is widespread on the eastern margin of the Kaapvaal Craton, but its origin is still poorly understood, mainly because geochemical data is rare. To shed some light on the source of this Mesoarchean magmatism and to relate different Mesoarchean volcanic sequences to each other, we provide major and trace element data as well as Hf-Nd isotope compositions of amphibolites sampled close to the Kubuta Ranch in south-central Eswatini. These amphibolites, so far, were of unknown correlation to any volcanic sequence in Eswatini or South Africa. Hence, the aim of our study is to characterize the mantle source composition of these volcanic rocks and, furthermore, to constrain their genetic relation to other volcanic sequences in Eswatini and South Africa. Our findings reveal that, based on coherent trace element patterns and similar Nd isotope characteristics, the Kubuta volcanic rocks can be genetically linked to the ca. 3.0 Ga Usushwana Igneous Complex in West-Central Eswatini and the ca. 2.9 Ga Hlagothi Complex located in the KwaZulu-Natal province. In contrast, the coeval ca. 3.0 Ga Nsuze and ca. 2.9 Ga Mozaan Groups (Pongola Supergroup) of south-central Eswatini and northern KwaZulu-Natal province have slightly enriched compositions compared to the newly sampled Kubuta volcanic rocks. Our results suggest that the Nsuze and Mozaan Groups were sourced from a primitive mantle reservoir, whereas the Usushwana, Hlagothi, and Kubuta mafic rocks were derived by melting of a more depleted mantle source comparable to that of modern depleted MORB. Furthermore, our assimilation-fractional crystallization (AFC) calculations and Nd isotope constraints reveal that some samples were contaminated by the crust and that the crustal contaminants possibly represent felsic rocks related to the ca. 3.5 Ga crust-forming event in the Ancient Gneiss Complex. Alternatively, melting of a metasomatized mantle or plume-lithospheric mantle interaction may also produce the trace element and isotopic compositions observed in the samples. From a synthesis of our geochemical observations and age data from the literature, we propose a refined petrogenetic model, for a continental flood basalt setting in a Mesoarchean large igneous province on the eastern Kaapvaal Craton. Our petrogenetic model envisages two magma pulses sourced from a primitive mantle reservoir that led to the formation of the Nsuze (first) and Mozaan (second) lavas. Conductive heating of ambient depleted mantle by the mantle plumes caused partial melting that led to the formation of the Usushwana Igneous Complex associated with the first magmatic event (Nsuze) and the Hlagothi Igneous Complex associated with the second magmatic event (Mozaan). However, due to lacking age data of sufficient resolution, it is not possible to affiliate the Kubuta lavas to either the first or the second magmatic event.

Detrital zircon provenance of the Archean Moodies Group, Barberton Greenstone Belt, South Africa and Eswatini

Sandstones of the 3.22 Ga Archean Moodies Group represent one of the world9s oldest quartz-rich sedimentary sequences. The provenance of this unit is unresolved because its quartz and common microcline can be sourced both from either now eroded or covered granitoid plutons outside the Barberton Greenstone Belt (BGB) or, alternatively and perhaps more controversially, (rhyo-)dacitic (sub-)volcanic rocks within the BGB. We compiled 31 detrital zircon data sets (n = 2588) from sandstones, reworked tuffs and conglomerate of the Moodies Group in order to constrain its age and provenance. After selection using quality criteria, the remaining zircons (n = 1621) in nearly all samples show a distribution corresponding to the four known major pulses of felsic magmatism in the BGB: 1) the <i>ca.</i> 3550 to 3530 Ma Theespruit and Sandspruit Formations at the base of the Onverwacht Group; 2) the <i>ca.</i> 3440 to 3410 Ma rhyo-dacites of the upper Hooggenoeg Formation, Onverwacht Group; 3) the <i>ca.</i> 3300 to 3280 Ma thin felsic tuffs in the Mendon Formation; and 4) the <i>ca.</i> 3260 to 3215 Ma felsic volcanic and shallow intrusive rocks of the Auber Villiers, Bien Venue and Schoongezicht Formations of the upper Fig Tree Group and the Moodies Group as well as their co-magmatic plutonic counterparts. Almost all data sets also contain near-concordant younger zircons as young as 2820 Ma, which can be attributed to one of six tectonic or magmatic events affecting the young Kaapvaal Craton in post-BGB time, causing the partial or complete resetting of the U-Pb system in some grains. The youngest (near-)concordant zircon clusters yield ages of <i>ca.</i> 3220 Ma in most locations, and the youngest discordant group of zircons from a reworked tuff near the top of the Moodies Group at 3212 ± 13 Ma agrees well with previous estimates of the maximum depositional age. With very few exceptions, the oldest zircons (<i>ca.</i> 3564 Ma) are only slightly older than the oldest rocks in the BGB stratigraphy (<i>ca.</i> 3550 Ma). Subtle regional and stratigraphic differences in age spectra may indicate localized or nearby sediment sources within a synorogenic setting. Preliminary age spectra along vertical stratigraphic profiles show little systematic variation, possibly indicating that intrabasinal recycling was dominant over considerable time periods of the basin9s evolution. Extra-basinal plutonic sources of similar age and composition as the intra-BGB sources appear to be required to provide quartz and some feldspar to Moodies Group sandstones, although zircon age spectra, limited zircon Hf isotope data, sandstone petrography, facies analysis, and the high variability in Moodies conglomerate clast composition are consistent with uplift, deformation and erosion of several intra-BGB sources, but in particular from the region of the Onverwacht Anticline. Zircon populations, conglomerate clasts, and sandstone composition show no evidence that high-grade metamorphic rocks from the adjacent Ancient Gneiss Complex (AGC) contributed significantly to the Moodies Group.

Archean crustal growth and reworking revealed by combined U-Pb-Hf-O isotope and trace element data of detrital zircons from ancient and modern river sediments of the eastern Kaapvaal Craton

It is intriguing to speculate on how continental crust grew and differentiated during the Archean and which tectonic regimes and magmatic mechanisms were responsible for such a process. The continental nucleus of the Kaapvaal Craton, which includes the Barberton granitoid-greenstone terrane (BGGT) in South Africa and the Ancient Gneiss Complex (AGC) in Swaziland, exhibits the best preserved geological record from 3.6 to 3.0 Ga and thus provides the optimal location to address this issue. In this study, combined U-Pb, O and Lu-Hf isotope and trace element analyses were carried out on detrital zircon grains from two Moodies Group sandstones and three modern river sand samples in the Barberton and Swaziland regions, southern Africa. The results, in conjunction with previously published data, show that the detrital zircons in the Moodies sandstones on both sides of the Inyoka Fault, had two similar U-Pb age clusters of 3.25–3.30 Ga and 3.40–3.47 Ga and the oldest age up to 3.57 Ga. This suggests that they were derived from older felsic volcanic rocks of the greenstone belt and the granitoid gneisses in the BGGT and AGC. Using the approach created for Earth’s oldest zircons (i.e., Jack Hills detrital zircons), we inferred that the source magmas of the studied detrital zircons had average arc-like andesitic compositions (SiO2 = 58.0 ± 5.6 wt.%; Th/Nb = 5.0 ± 3.2), similar to those of the Jack Hills detrital zircons. However, these inferred compositions conflict with the source rocks of the detrital zircons, namely, the felsic volcanic rocks and the granitoid gneisses in the BGGT and AGC, which have high SiO2 contents (average = 70 ± 4.3 wt.%) and moderate Th/Nb ratios (average = 0.76 ± 0.44). These findings suggest that the inferred average andesitic compositions for the Earth’s oldest zircons should be reevaluated. The U-Pb age spectra of detrital zircons in modern river sands suggest that the exposed rocks in southwestern Swaziland are dominantly ca. 3.45 Ga tonalitic, trondhjemitic and granodioritic (TTG) gneisses without Eoarchean crust. The concordant zircon U-Pb age data reveal four major magmatic episodes for the eastern Kaapvaal Craton at 3.52, 3.46, 3.26 and 3.10 Ga. The first magmatic episode is suggested to dictate crustal growth, whereas the other three magmatic episodes are predominated by crustal reworking. This process is manifested by major positive changes in εHf(t) and εNd(t) values starting at ∼3.5 Ga, gradual decreases in εHf(t) and εNd(t) values afterwards, and increases in δ18O values at ∼3.25 Ga. Combined with other geological observations, we suggest that modern-style plate tectonic processes, such as subduction, may not have been a requirement for crustal growth and reworking in the eastern Kaapvaal Craton during the Paleoarchean (3.6–3.2 Ga). Instead, they could be ascribed to episodic partial convective overturns at different crustal depths.

Palaeoarchaean TTGs of the Pilbara and Kaapvaal cratons compared; an early Vaalbara supercraton evaluated

AbstractThe continental crust that dominates Earth’s oldest cratons comprises Eoarchaean to Palaeoarchaean (4.0 to 3.2 Ga) felsic intrusive rocks of the tonalite-trondhjemite-granodiorite (TTG) series. These are found either within high-grade gneiss terranes, which represent Archaean mid-continental crust, or low-grade granite-greenstone belts, which represent relic Archaean upper continental crust. The Palaeoarchaean East Pilbara Terrane (EPT), Pilbara Craton, Western Australia, and the Barberton Granite-Greenstone Belt (BGGB), Kaapvaal Craton, southern Africa, are two of the best exposed granite-greenstone belts. Their striking geological similarities has led to the postulated existence of Vaalbara, a Neoarchaean-Palaeoproterozoic supercraton. Although their respective TTG domes have been compared in terms of a common petrogenetic origin reflecting a volcanic plateau setting, there are important differences in their age, geochemistry, and isotopic profiles. We present new zircon Hf isotope data from five granite domes of the EPT and compare the geochemical and isotopic record of the Palaeoarchaean TTGs from both cratons. Rare &amp;gt;3.5 Ga EPT evolved rocks have juvenile εHf(t) requiring a chondritic source. In contrast, younger TTG domes developed via 3.5 to 3.4 and 3.3 to 3.2 Ga magmatic supersuites with a greater range of εHf(t) towards more depleted and enriched values, trace element signatures requiring an enriched source, and xenocrystic zircons that reflects a mixed source to the TTGs, which variously assimilates packages of older felsic crust and a more juvenile mafic source. EPT TTG domes are composite and record multiple pulses of magmatism. In comparison, BGGB TTGs are less geochemically enriched than those of the EPT and have different age profiles, hosting coeval magmatic units. Hafnium isotopes suggest a predominantly juvenile source to 3.2 Ga northern Barberton TTGs, limited assimilation of older evolved crust in 3.4 Ga southern Barberton TTGs, but significant assimilation of older (Hadean-Eoarchaean) crust in the ca. 3.6 Ga TTGs of the Ancient Gneiss Complex. The foundation of the EPT is younger than that for the oldest components of the Eastern Kaapvaal. Although the broader prevailing Palaeoarchaean geologic framework in which these two cratons formed may reflect similar a geodynamic regime, the superficial similarities in dome structures and stratigraphy of both cratonic terranes is not reflected in their geochemical and age profiles. Both the similarities and the differences between the crustal histories of the two cratons highlights that they are formed from distinct terranes with different ages and individual evolutionary histories. Vaalbara sensu lato represents typical Palaeoarchaean cratonic crust, not in the sense of a single homogeneous craton, but one as diverse as the continents are today.

Hafnium-Neodymium isotope, trace element and U-Pb zircon age constraints on the petrogenesis of the 3.44–3.46 Ga Dwalile greenstone remnant, Ancient gneiss Complex, Swaziland

The geodynamic setting of the 3.44–3.46 Ga Dwalile greenstone remnant (DGR) of the Ancient Gneiss Complex (Swaziland) and its relationship to other Greenstone belts of similar age is investigated in this contribution. We present Hf-Nd isotope, major and trace element data for komatiites, komatiitic basalts, basalts, basaltic andesites as well as felsic volcanic and sedimentary rocks, all metamorphosed at amphibolite-facies grade. Major and trace element compositions of metabasalts and metakomatiites indicate crustal contamination. The best-preserved samples yield 176Lu-176Hf and 147Sm-143Nd isochron ages of ca. 3500 and 3460 Ma, respectively, in line with U-Pb zircon ages of interlayered felsic volcanic rocks of 3460 Ma. Isotope and trace element data for the metasedimentary rocks reveal a contribution of older crustal material with 3.53 Ga zircon grains. Together with >3.7 Ga inherited zircon grains in the Ngwane gneiss (NG) of the basement, this evidence supports a model of deposition of the Dwalile supracrustal rocks on or near older continental crust. Initial εHf and εNd values of the least contaminated DGR metabasalts and metakomatiites are similar to those of the contemporaneous Komati Formation komatiites and tholeiitic basalts, however, the trace element composition and slight correlations of initial εHf and εNd values with major and trace element parameters indicative for crustal contamination reveal differences in the basement of both units.

Extensive magmatism and metamorphism at ca. 3.2 Ga in the eastern Kaapvaal Craton

The timing of the emergence of modern-style plate tectonics on Earth is of fundamental importance in understanding the thermal and compositional history of the planet. Although the magmatism and metamorphism in the eastern Kaapvaal Craton was thought by some as geological records for mid-Archean subduction at 3.2 Ga, their petrogenesis was fiercely debated. To reveal the nature of the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton, here we provide a comprehensive zircon and monazite U-Pb geochronological investigation, coupled with in situ trace element and Hf-O isotopic analyses, for the magmatic and metamorphic rocks from the Barberton granitoid-greenstone terrane in South Africa and Ancient Gneiss Complex in Swaziland. In spite of deriving from potentially different sources as revealed by different lithology, nine magmatic rocks sampled from different plutons have consistent zircon/monazite SIMS 207Pb/206Pb ages of 3240–3220 Ma. Distinct zircon grains, including inherited magmatic and metamorphic, are identified in the metamorphic rocks by detailed CL images and U-Pb geochronological, trace elemental and Hf-O isotopic analyses. The metamorphic zircons give consistent 207Pb/206Pb ages of 3240–3220 Ma. One granitic gneiss and a metapelite give monazite SIMS 207Pb/206Pb dates of 3230.6 ± 2.7 Ma and 3223.1 ± 2.4 Ma, respectively, which are consistent with the results of metamorphic zircon and interpreted as the age of metamorphism. Most metamorphic samples give Ti-in-zircon temperatures of 650–740 °C and one sample yields Y-in-monazite temperatures of 918–966 °C, which imply that the rocks of AGC were universally metamorphosed to upper amphibolite facies, and locally reaching high to ultrahigh temperature (HT-UHT) granulite facies (>900 °C) at 3240–3220 Ma. This study, in combination with previous results, suggests widespread distribution for the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton without asymmetry in the thermal structure, which is quite different from those of the modern subduction zones. The synchronous occurrence of the metamorphism and magmatism in the eastern Kaapvaal Craton at 3.2 Ga was interpreted as the results of partial convective overturn of the crust.

Comment on “High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa” by Kröner et al.– why heating of the Ancient Gneiss Complex by a mantle plume at 3.2 Ga is not a viable tectonic model

Comment on “High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa” by Kröner et al.– why heating of the Ancient Gneiss Complex by a mantle plume at 3.2 Ga is not a viable tectonic model

High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa

The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66–3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane.Some previous models have related the 3.2 Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2 Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.

Coexistence of enriched and modern-like 142Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa

The short-lived 146Sm–142Nd isotope system is an important tool for tracing Hadean crust–mantle differentiation processes and constraining their imprint on much younger rocks from Archean cratons. We report the first comprehensive set of high-precision 142Nd analyses for granitoids and amphibolites of the Ancient Gneiss Complex (AGC; Swaziland) and the oldest metavolcanic units of the Barberton Greenstone Belt (BGB; South Africa). The investigated samples span an age range from 3.66 Ga to 3.22 Ga and are representative of major geological units of the AGC and the lower Onverwacht Group of the BGB. Measured samples yielded μ142Nd values in the range from −8 ppm to +3 ppm relative to the JNdi-1 terrestrial standard, with typical errors smaller than 4.4 ppm. The distribution of the μ142Nd values for these 17 measured samples is bimodal with ten samples showing a tendency towards slightly negative μ142Nd anomalies, whereas seven samples have 142Nd similar to the terrestrial reference. The only confidently resolvable μ142Nd anomalies were found in a 3.44 Ga Ngwane Gneiss sample and in amphibolites of the ca. 3.45 Ga Dwalile Greenstone Remnant, revealing μ142Nd values ranging from −7.9±4.4 to −6.1±4.3 ppm. The μ142Nd deficits do not correlate with age, lithological unit, or sample locality. Instead, our results reveal that two distinct mantle domains were involved in the formation of the AGC crust. The two reservoirs can be distinguished by their μ142Nd signatures. Mantle-derived rocks tapped the enriched reservoir with negative μ142Nd at least until 3.46 Ga, whereas the granitoids preserved a negative μ142Nd signature that formed by incorporation of older AGC crust at least until 3.22 Ga. The oldest gneisses with no μ142Nd anomaly are up to 3.64 Ga in age, indicating that a modern terrestrial 142Nd reservoir was already present by early Archean times.

Zircon geochronology and Hf isotopes of the Dwalile Supracrustal Suite, Ancient Gneiss Complex, Swaziland: Insights into the diversity of Palaeoarchaean source rocks, depositional and metamorphic ages

We report the results of combined U-Pb and Lu-Hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano-sedimentary rocks of the Dwalile Supracrustal Suite (DSS) from the ca. 3.66 to 3.20Ga Ancient Gneiss Complex (AGC) in Swaziland. The results indicate that the DSS is made up by Palaeo- and Mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the Neoarchaean. The oldest Dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca. 3.46Ga, and are derived from the same magma source as the Tsawela Gneisses. Based on the combined U-Pb-Hf isotope data, two groups of metasedimentary rocks can be distinguished: an older Dwalile Group I that is similar in age to the Hooggenoeg and Kromberg Formations (3.46–3.40Ga), but with a different source area. A younger Dwalile Group II is coeval with the Fig Tree and possibly Moodies Groups (<3.23Ga) and the data suggest there was a similar provenance.The U-Pb-Hf isotope data and petrological observations demonstrate that the DSS was affected by three stages of metamorphic overprint: an event at 3.23Ga, pervasive amphibolite-facies metamorphism at 3.15Ga, and low-P granulite-facies metamorphism at 2.99Ga. Metamorphic zircon in the amphibolite-facies rocks resulted from new growth in aqueous fluids at 3.15Ga, whereas in the granulite-facies rocks new zircon was formed in the presence of melt. The 3.23Ga event coincides with the last significant metamorphic overprint documented in the Barberton Greenstone Belt (BGB), whereas as two younger events are restricted to the AGC.The data support the model suggesting amalgamation between the AGC and BGB terranes at ca. 3.23Ga. In addition, they require crustal thickening in the AGC terrane at ca. 3.15Ga, causing Barrovian-type metamorphism in the DSS and granulite-facies metamorphism in the Luboya-Kubuta Terrane. Low-P granulite-facies metamorphism at ca. 2.99Ga is most likely related to crustal extension during the onset of Pongola Basin rifting, causing mantle upwelling, mafic magmatism, and heat transfer from the mantle into the thinned crust.

Erosion of Archean continents: The Sm-Nd and Lu-Hf isotopic record of Barberton sedimentary rocks

Knowing the composition, nature and amount of crust at the surface of the early Earth is crucial to understanding the early geodynamics of our planet. Yet our knowledge of the Hadean-Archean crust is far from complete, limited by the poor preservation of Archean terranes, and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants. To address this problem and get a more comprehensive view of what an Archean continent may have looked like, we investigated the trace element and Sm-Nd, Lu-Hf isotopic records of Archean metasedimentary rocks from South Africa. We focused our study on sandstone and mudstone from drill core in the Fig Tree Group (3.23–3.26Ga) of the Barberton granite-greenstone belt, but also analyzed the 3.4Ga Buck Reef cherts and still older (3.5–3.6Ga) meta-igneous rocks from the Ancient Gneiss Complex, Swaziland.Based on principal component analysis of major and trace element data, the Fig Tree metasedimentary rocks can be classified into three groups: crustal detritus-rich sediments, Si-rich sediments and Ca-, Fe-rich sediments. The detritus-rich sediments have preserved the Sm-Nd and Lu-Hf isotopic signatures of their continental sources, and hence can be used to constrain the composition of crust eroded in the Barberton area in the Paleoarchean period. Based on Sm/Nd ratios, we estimate that this crust was more mafic than today, with an average SiO2 content of 60.5±2wt.%. This composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic-ultramafic and felsic rocks. This implies that the Archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth. Neodymium and Hf crustal residence ages show that the eroded crust was, on average, ∼300–400Ma older than the deposition age of the sediments, which highlights the importance of intracrustal reworking of older crust at ∼3.2Ga in the Barberton area.The Si-rich sediments have slightly positive εNd (t=3.23Ga) but extremely radiogenic εHf (t=3.23Ga), up to +11. Based on analyses of 3.4Ga Buck Reef cherts, we suggest that the radiogenic Hf isotopic signature of the Si-rich sediments can be accounted for by the old chert clasts or detrital silicified rock fragments present in the rocks. The latter have extremely high Lu/Hf ratios such that their εHf values would increase dramatically, by about +100 epsilon units every 100Ma. In the Ca-, Fe-rich sediments, one sample contains carbonate that preserves the typical rare-earth element features of seawater precipitates. The initial Nd isotopic composition of this sample (εNd (t=3.23Ga)= +1.7) is within the range of previous estimates for Archean anoxic seawater.

U–Pb–Hf isotope systematics of detrital zircons in high-grade paragneisses of the Ancient Gneiss Complex, Swaziland: Evidence for two periods of juvenile crust formation, Paleo- and Mesoarchaean sediment deposition, and 3.23 Ga terrane accretion

We present the first results of combined U–Pb and Lu–Hf isotope analyses of detrital zircon grains, in high-grade paragneisses (Luboya-Kubuta granulites) from the ca. 3.66–3.22Ga Ancient Gneiss Complex (AGC) in Swaziland. Uranium–Pb ages demonstrate that sedimentation along the SE margin of the proto-Kaapvaal Craton took place during two time intervals: during the Paleoarchaean ca. 3.53 and >3.43Ga; and Mesoarchaean ca. 3.22 and >3.16Ga. The Hf-isotope data show that the hinterland to the Paleoarchaean metasediments experienced juvenile crust formation from a depleted mantle source at ca. 3.53Ga (ƐHf3.53Ga values between −0.7 and +4.1), and by reworking of more ancient crust (TDM=3.8–3.6Ga). In contrast, detrital zircons in Mesoarchaean metasediments record juvenile crust formation at 3.32Ga (ƐHf3.32Ga values between −1.5 and +6.0; TDM=3.45–3.32Ga), followed by internal reworking of this crust until 3.22Ga. A small number of detrital zircons indicate limited input from older, Paleoarchaean AGC crust. Compared to sediments of the Barberton Greenstone Belt, i.e. Fig Tree and Moodies Group, the Mesoarchaean metasediments of Swaziland are more juvenile in character, and show greater complexity in their detrital zircon age spectra, pointing towards a different provenance. Our new U–Pb–Hf isotope data, together with field and petrological information from the study area, suggest that the Paleoarchaean metasediments were largely derived from the AGC; whereas the Mesoarchaean metasediments were derived from a ca. 3.45–3.32Ga juvenile island-arc terrane (located SE of the AGC), which became accreted onto the Kaapvaal Craton at ca. 3.23Ga, i.e. the onset of burial and high-grade metamorphism in the area.

Source composition, fractional crystallization and magma mixing processes in the 3.48–3.43 Ga Tsawela tonalite suite (Ancient Gneiss Complex, Swaziland) – Implications for Palaeoarchaean geodynamics

The ca. 3480–3430Ma Tsawela Gneiss (TG) is a well-preserved coarse-grained diorite to tonalite suite within the Ancient Gneiss Complex of Swaziland, eastern Kaapvaal craton. These gneisses are texturally and compositionally distinct from the hosting oldest components of the ca. 3200–3660Ma TTG-type Ngwane Gneisses (NG). Major and trace elements, in combination with whole-rock hafnium-neodymium isotopic data, were analyzed in the TG and in three samples of ca. 3450Ma grey NG to constrain sources and magmatic processes. High-field-strength element data (HFSE) were combined with U-Pb SHRIMP II ages and Hf-in-zircon data for key samples to constrain their ages and petrogenesis.In contrast to the widespread view that Archaean crust is mainly composed of TTG igneous suites that formed from juvenile sources, the geochemical and isotopic compositions of the TG indicate that these rocks represent a calc-alkaline plutonic suite which possibly formed by magma mixing processes involving juvenile, mantle-derived tholeiitic melts as well as partial melts of the older Ngwane gneiss. Alternatively, the TG may represent a magmatic suite that formed by fractional crystallization of a hydrous intermediate magma. These results contrast with field evidence of a relatively uniform and homogeneous composition. Our geochronological and isotopic data show that the TG intruded the NG during a time span of at least 50Ma without any significant compositional change of the source. The predominant influence of fractional crystallization of a tholeiitic mafic magma, as well as assimilation-fractional-crystallization processes (AFC) can be excluded for the TG from major and trace element modelling. The magma processes proposed here suggest efficient mixing of approximately equal amounts of TG magmas with those derived from the NG basement and is supported by the largely homogeneous Hf-Nd isotopic compositions of the whole-rock samples.We propose that melting and mixing occurred in the lower crust that mainly consisted of >3.50Ga NG and was possibly triggered by plume-related underplated and intraplated tholeiitic magmas in sills and/or laccoliths. This model is supported by geochemical evidence for a mafic end member lacking a negative Nb anomaly and implying that subduction processes were not involved in the formation of the TG.

Generation of early Archaean grey gneisses through melting of older crust in the eastern Kaapvaal craton, southern Africa

We report zircon ages, Hf-in-zircon isotopes as well as whole-rock geochemistry and Hf–Nd isotopic systematics for Palaeoarchaean grey gneisses of the Ancient Gneiss Complex of Swaziland, the oldest components of the Kaapvaal craton, southern Africa. The Hf-in-zircon isotopic compositions in these compositionally heterogeneous, multicomponent, migmatitic gneisses are highly variable, even in the oldest zircons dating back to 3.66Ga, suggesting growth of zircon from melts representing a mix of juvenile and anatectic material derived from differentiated continental crust of Eoarchean to late Hadean age. In contrast, the initial Nd and Hf whole-rock isotopic compositions are frequently not in agreement with the Hf-in-zircon data that mostly show approximately chondritic initial values for Nd and strongly radiogenic initial values for Hf. We consider it likely that both the Lu–Hf and Sm–Nd whole-rock isotopic systems were disturbed and partly reset during later episodes of partial melting and crustal reworking, most likely during a pervasive 3.2Ga tectono-metamorphic event.Primitive mantle-normalized trace element patterns show the variable influence of residual plagioclase and garnet in the sources as well as high contents of strongly incompatible elements. In conjunction with the Hf-in-zircon isotopic data the trace element contents are best explained by the incorporation of older continental crustal material into the sources of the grey gneisses. Our data support evidence from other Palaeoarchean terranes that crustal recycling, as seen in even the oldest crustal components, played an important role in early continental evolution. Rocks previously classified as a subduction-related tonalite–trondhjemite–granodiorite suite are complex, and their chemistry alone cannot be used to reconstruct tectonic settings.

AGE AND FIELD RELATIONSHIPS OF MAHAMBA ORTHOGNEISSES AND MKHONDO VALLEY METAMORPHIC SUITE PARAGNEISSES FROM THE MKHONDO RIVER, ANCIENT GNEISS COMPLEX, SWAZILAND.

This study documents for the first time the ages and structural field relationships of banded orthogneisses, and associated high-grade paragneisses, from the Mkhondo River in the Ancient Gneiss Complex of Swaziland, whose age and regional context has been disputed. LA-ICP-MS U-Pb zircon dating shows that the orthogneiss unit mapped as Mahamba gneiss is composed of a 3215 ± 12 Ma granodiorite phase that was intruded by a 3162 ± 11 Ma felsic granitic phase. A hornblende-bearing tonalite dated at 2889 ± 13 Ma, which occurs structurally beneath the Mkhondo Valley Metamorphic Suite (MVMS) metasediments, experienced a metamorphic overprint at 2735 ± 11 Ma that coincided with granulite facies metamorphism and partial melting in the MVMS (Taylor et al., 2010). Both the MVMS and the Mahamba gneisses are characterised by north-northwest aligned, syn-peak-metamorphic deformation structures, indicating a period of west-southwest to east-northeast directed crustal shortening. Neither the Mahamba gneisses, nor the hornblende tonalite are intrusive into the structurally overlying MVMS sediments. This suggests that: (1) the Mahamba gneisses form part of the exposed basement in the area; and (2) the MVMS is a relatively young, late Archaean sedimentary succession that is unrelated to ≥ 3.23 Ga high-grade paragneiss units documented to the east and northeast of the MVMS (Taylor et al., 2012).

Paleoarchean detrital zircon ages from the earliest tectonic basin in the Barberton Greenstone Belt, Kaapvaal craton, South Africa

Detrital zircon grains in coarse-grained sedimentary rocks of the Noisy formation, (traditionally considered as the H6 unit of the uppermost Hooggenoeg Formation) in the Paleoarchean Barberton Greenstone Belt (BGB) South Africa, are investigated by laser ablation (LA)-ICP-MS to constrain their U–Pb and 207Pb/ 206Pb ages for depositional age and provenance determination. Drill core and field observations indicate that these rocks are clastic polymictitic diamictites and sandstones. A wide range in 207Pb/ 206Pb ages between ca. 3600 and 3430 Ma is reported. The youngest detrital zircon grain identified has an age of 3432 ± 10 Ma, constraining the maximum depositional age of the sedimentary sequence and the formation of a major underlying erosional unconformity. The youngest grains correspond in age to the ca. 3460–3430 Ma tonalite–trondjemite–granodiorite (TTG) gneisses of the Stoltzburg terrane and felsic intrusions of the uppermost Hooggenoeg Formation that constitute most of the Songimvelo Block of the BGB on the Kaapvaal craton. The 207Pb/ 206Pb ages of the oldest grains identified corresponds to plutonic sources identified in older proto-continental blocks of the Kaapvaal craton, namely the ca. 3509 Ma Steynsdorp Block and ca. 3667–3223 Ma Ancient Gneiss Complex (AGC). This new data indicates provenancial linkage between the Songimvelo Block and older proto-continental blocks exposed on the Kaapvaal craton at ca. 3432 Ma. It is argued here, that the Noisy formation is a remnant of the earliest tectonic basin in the BGB, that developed during major tectonic accretion and uplift at ca. 3432 Ma that lead to formation of the erosional unconformity. In conjunction with new drill core and field observations, the geochronological constraints presented here support plate-tectonic processes operating in the proto-Kaapvaal craton that resulted in its formation as a stable continental landmass as early as ca. 3432 Ma.

Hafnium isotope record of the Ancient Gneiss Complex, Swaziland, southern Africa: evidence for Archaean crust–mantle formation and crust reworking between 3.66 and 2.73 Ga

Abstract: Combined U–Pb and Lu–Hf isotope analyses of zircons from 16 tonalite–trondjemite–granodiorite (TTG) gneiss and granite samples from Swaziland reveal that the oldest rocks of the Ancient Gneiss Complex in southern Africa formed by reworking of Early Archaean or perhaps Late Hadean crust at 3.66 Ga, and that new crust was extracted from a depleted mantle source during Palaeoarchaean events between 3.54 and 3.32 Ga. This interpretation is supported by εHf t of −1.6 ± 2.0 obtained from 3.66 Ga TTG gneisses, corresponding to hafnium model ages between 3.77 ± 0.18 Ga, for a presumed Hadean–Early Archaean chondritic mantle, and 4.08 ± 0.18 Ga, for a presumed Hadean depleted mantle reservoir, with the first model age being the most likely in the light of recent data from worldwide sources. Furthermore, it is reflected by superchondritic εHf t up to +2.2 ± 2.0 for TTGs formed at 3.54, 3.45 and 3.32 Ga. The new datasets additionally show that the Palaeoarchaean crust formed between 3.54 and 3.32 Ga was intensely reworked afterwards, without significant addition of depleted mantle derived material, during orogenic and intracratonic melting processes at 3.23, 3.1 and 2.7 Ga. This is well reflected by an array of decreasing εHf t from +2.2 to −7.2 between 3.3 and 2.7 Ga, which can be forced by 176 Lu/ 177 Hf of 0.0113, which is similar to that of present-day average continental crust, and might result from lower crust zircon fractionation during Archaean crust reworking. Supplementary material: Results of in situ U–Pb and Lu–Hf isotope zircon analyses and concordia diagrams are available at www.geolsoc.org.uk/SUP18465 .

Granulite facies anatexis in the Ancient Gneiss Complex, Swaziland, at 2.73 Ga: Mid-crustal metamorphic evidence for mantle heating of the Kaapvaal craton during Ventersdorp magmatism

The 3.7–3.2 Ga Ancient Gneiss Complex (AGC) of Swaziland, located S and SE of the Barberton Greenstone Belt (BGB), is a complex geological terrane where few studies have focussed on the timing and conditions of deformation and metamorphism, despite the fact that minor metasedimentary units within the AGC provide excellent opportunities for constraining metamorphic evolution. One of these, the Mkhondo Valley Metamorphic Suite consists of migmatitic metapelitic granulites, within which the peak metamorphic assemblage garnet + cordierite + biotite + plagioclase + quartz ± K-feldspar + melt is dominant. NCKFMASHTO pseudosection modelling of this anatectic assemblage constrains peak metamorphic conditions to 850–830 °C and 4.4 kbars, followed by near isobaric cooling of the terrane to 680 °C and 3.9 kbars. These conditions indicate a period of anomalously high heat flow within the crust due to the direct addition of mantle heat. U–Pb SHRIMP dating of zircons from an in situ leucosome indicates an age of ca. 2.73 Ga for peak granulite facies metamorphism, approximating the age of Ventersdorp Supergroup flood basalt volcanism on the craton and widespread intracratonic granitic magmatism. The current study suggests that substantial heat input to the Kaapvaal cratonic crust during underplating by Ventersdorp related magmas, possibly related to the presence of a ∼2.7 Ga mantle plume marginal to the craton, was responsible for high-grade metamorphism in the AGC and a period associated with regional crustal anatexis. The resultant mid-crustal ductility and weakening may have initiated mobilisation of the mid-crust as gneissic domes, producing basement uplift and sedimentary recycling of auriferous reefs of the Central Rand Group of the Witwatersrand Supergroup.

Rates and mechanisms of Mesoarchean magmatic arc construction, eastern Kaapvaal craton, Swaziland

The mechanisms and time scales of magmatism and deformation during orogenesis are important for developing models of lithospheric growth. We summarize the first detailed study of a well-preserved Mesoarchean crustal section from the oldest portion of the Kaapvaal craton, southern Africa, which records the complex interactions between deformation and magmatism during craton assembly ca. 3.3–3.2 Ga. We use chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology and apatite thermochronology in conjunction with geological mapping to show that the tonalitic to granitic Usutu magmatic suite intruded into the ca. 3.66–3.45 Ga Ancient gneiss complex over a period of ∼16 Ma ca. 3236–3220 Ma as discrete pulses of magma with variable intrusive styles. Usutu rocks retain magmatic fabrics that preserve a history of NW-SE regional shortening, consistent with synchronous deformation recorded in the adjacent Barberton greenstone belt. U-Pb zircon dates of ca. 3.28–3.23 Ga and apatite cooling dates from the Nhlangano gneiss SE of the Ancient gneiss complex reveal that they represent an older, largely undocumented period of crustal growth and modification. Synchronicity in magmatism and similarity in kinematics of deformation north and south of a previously suggested continental suture in the Barberton greenstone belt lead us to propose doubly vergent subduction zones that were active from at least ca. 3.28 to 3.22 Ga. Geochemistry of the Usutu suite reveals differences in magma sources from north to south, which, when combined with Nd isotopic signatures, are consistent with their production in a subduction zone. The combination of <1 Ma precision on crystallization dates with field observations and geochemistry allows us to track the evolution of this magmatic system with temporal resolution unprecedented for Archean rocks.